Current transformer



P 1, 1964 s. GALLOUSIS CURRENT TRANSFORMER Filed May 8, 1961 United States Patent This invention relates to a means for enclosing electrical apparatus and in particular to through type current transformers.

One of the past methods for housing instrument transformers including current and potential transformers Was to surround the core and winding of the transformer with insulating material such as tape and then mount the unit Within a metal container. Bushings were then clamped to the metal container with the bushings generally being of a ceramic material.

However, with the advance in synthetic material technology in recent years, there has been a trend toward encapsulating instrument transformers with a dielectric synthetic moldable material usually of the thermosetting variety. The bushings have frequently been made of the same synthetic material. In addition to cost advantages, these synthetic materials have good resistance to mechanical shocks and weather.

Another arrangement for housing current transformers hasbeen to combine a ceramic bushing with the encapsulated transformer windings and cores. Although ceramics are more brittle than conventional thermosetting materials, many ceramics such as porcelain have greater dielectric characteristics. Presently, manufacturers of transformers utilizing a combination of thermosetting resins and porcelain generally mount the ceramic bushings onto the encapsulated units with conventional fastening hardware. Because of the shrinking characteristics of thermosetting materials, attempts to mold ceramic bushings into plastic casings have not been too successful. The large mass of thermosetting material, as it cures, is apt to stress and crack the bushing. The invention described herein provides a highly Weather resistant current transformer having exceptionally good dielectric qualities through the combination of an encapsulated winding and core combined with a porcelain bushing without the aid of clamping and fastening accessories.

Thus, it-is one object of this invention to provide a new and improved transformer having excellent weather resistant properties.

Another object of this invention is to provide a new and improved transformer which utilizes the best qualities of ceramics and plastics.

Objects and advantages other than those mentioned above will be apparent upon reading the following detailed description in connection with the drawing in which:

FIG. 1 is a front view in elevation'with a portion broken away of a transformer embodying the features of this invention; and

FIG. 2 is a sectional view of the transformer taken along lines II-II of FIG. 1.

Referring to the drawing, the illustrated transformer 11 is generally known as a window or through type current transformer. The transformer has an encapsulated unit 13 comprising a core 15 and electrical secondary winding 16. The core is comprised of a continuously wound strip of magnetic sheet material. However, this invention also applies to transformers having cores with a plurality of wound strips or those with stacked laminations. The winding 16 may be of the toroidal type and is wound on the core. The core and winding are encapsulated with a layer 18 of a moldable electrical insulating material. Suitable materials include solid setting resins such as epoxy the encapsulated transformer.

3,147,456 Patented Sept. 1, 1964 resins, polyester resins and polyester modified epoxy resins.

In manufacturing the unit 13, comprising an encapsulated winding and core, the core 15 is first wound and annealed. If desired, a base assembly 21 may be spot Welded to the core. The secondary winding 16 is then wound on the core and terminals 23 (only one illustrated) are attached to the winding. A terminal housing 24 may be mounted over the terminals. The winding and core may now be placed in a conventional mold of the split type variety. The unit is carefully centered in the mold and a suitable insulating material is introduced in the mold to provide a uniform encapsulating layer 18 about the core and Winding. Two such materials are Araldite resin CN-SOS, sold by the Ciba Company, and Hysol resin 6800892, available at Houghton Laboratories, Inc. After a suitable amount of curing time, the encapsulated core and coil may be removed from the mold.

In order to prevent arcing and fiashover, a tubular bushing 25 of a ceramic material such as porcelain is mounted within the opening 27 of the encapsulated core and winding. Avoidance of hardware fastening elements is accomplished by molding the bushing in the opening of the encapsulated transformer with a second layer 29 of resin. The inside circumference of the transformer may be coated with a conventional grease mold release agent 31 such as Dow Corning release agent DC*7. Likewise, the outside circumference of the ceramic bushing may be coated with a layer 32 of the release agent. During a subsequent casting this resin coating provides a cushioning effect andalso a reduction and equalization of strains imposed on the ceramic bushing. The bushing is then centered within the opening of the transformer. In order to maintain uniform distance between the bushing and the transformer, the two elements may be positioned within a molding fixture (not shown). An insulating material is introduced into the space between the bushing 25 and encapsulated unit 13 and allowed to solidify and cure.

Because of the brittle nature of the bushing it is preferable to use a slightly resilient resin such as Scotchcast #2 resin sold by Minnesota Mining and Manufacturing Company. After a suitable amount of time the completed assembly may be removed from this mixture. Attention is called to the fact that two molding procedures have been involved in combining the ceramic bushing with Because of the relatively small amount of resin in the second layer 29 to be cured, shrinkage problems normally present in a single molding operation and which set up stresses on the ceramic bushing are substantially eliminated.

Referring to FIG. 2 of the drawing, it can be seen that at each end of the second layer 29 of resin there is a flared annular portion 33 overlapping the ends of the first layer 18. Each flared portion is complementary to a respective chamfered surface 35. The chamfered surface 35 may be made through the particular configuration of the moldused for encapsulating the unit. FIG. 2 also reveals how the portion of the bushing within the opening of the transformer has a raised surface 37. The purpose of the chamfered and flared portions on the ends of the two layers 13 and 29 is to provide an interlocking effect between the layers. The raised surface on the bushing further improves the interlocking effect and provides a better grip between the second layer of insulating end of the second layer of insulating material overlapping the chamfered Surface 35 of the first layer. After removing the flared portion around the entire periphery of one end of the second layer, the bushing and its bonded second layer may be removed from the transformer through the opposite end of the transformer opening. A very small amount of force is required to loosen the first layer from the second layer of insulating material. The film of mold release agent 31 aids in the separation of the two layers since it reduces the bond between them. Upon removal of the damaged bushing, a new bushing may be mounted in the core opening through operations similar to those described above for casting the second layer of insulating material.

While only one embodiment of the present invention has been illustrated and described it will be apparent to those skilled in the art that modifications other than those shown may be made without departing from the spirit of the invention or from the scope of the appended claims.

Having now particularly described and ascertained the nature of my said invention and the manner in which it is to be performed, I declare that what I claim is:

1. A transformer comprising: a core having an opening; an electrical winding linked with said core, a first layer of electrical insulating material encapsulating said core and winding; a bushing extending through said core opening; and a second layer of insulating material molded in contiguous relationship with said bushing and the portion of said first layer encircling said core opening.

2. A transformer comprising: a core having an opening; an electrical Winding toroidally linked with said core; a first layer of electrical insulating material encapsulating said core and winding; a ceramic tubular bushing coaxially extending through said core opening; and a second layer of insulating material molded between said bushing and core.

3. A transformer comprising: a core having an opening; an electrical winding linked with said core; a first layer of electrical insulating material encapsulating said core and winding; a bushing extending through said core opening; and a second layer of insulating material molded in contiguous relationship with said bushingand the portion of said first layer encircling said core opening; said second layer interlocked with said first layer at an end of said core opening.

4. A transformer comprising: a core having an opening; an electrical winding toroidally linked with said core; a first layer of electrical insulating material encapsulating said core and winding; a ceramic tubular bushing coaxially extending through said core opening; and a second layer of insulating material molded between said bushing and the portion of said first layer encircling said core opening to maintain the relative position of said bush- I ing and core, said second layer interlocked with said first layer at an end of said core opening.

5. A transformer comprising: a wound core having an opening; an electrical winding toroidally wound on said core; a first layer of electrical insulating material encapsulating said core and winding; a ceramic tubular bushing extending through said core opening; a second layer of insulating material molded in bonding relationship with said bushing and contiguous to the portion of said first layer encircling said core opening; and means interlocking said second layer with said first layer comprising flared portions on the ends of one of said layers overlapping portions of the ends of said other layer.

6. A transformer comprising: a wound core having an opening; an electrical winding toroidally wound on said core; a first layer of electrical insulating material encapsulating said core and winding; a ceramic tubular bushing extending through said core opening; a second layer of insulating material molded in bonding relationship with said bushing and contiguous to the portion of said first layer encircling said core opening; a thin film of a mold release agent between said first and second layers;

I I said second layer interlocked With said first layer at the ends of said core opening.

7. A transformer comprising: a wound core having an opening; an electrical winding toroidally wound on said core; an encapsulating first layer of electrical insulating material molded about said core and Winding; a ceramic tubular bushing coaxially extending through said core opening; a thin coat of grease covering the exposed surface of said first layer within said core opening; and a second layer of insulating material molded between said bushing and the coated portion of said first layer encircling said core opening to maintain the relative position of said bushing and core, said second layer interlocked with said first layer by means comprising flared ends on one of said layers extending over the ends of said other layer.

8. A transformer comprising: a wound core having an opening; an electrical winding toroidally wound on said core; an encapsulating first layer of electrical insulating material molded about said core and winding; a thin film of a mold release agent adhering to the surface of said first layer within said core opening; a ceramic tubular bushing coaxially extending through said core opening and spaced from said film and a second layer of insulating material molded into the remaining space between said bushing and said release agent film, said second layer interlocked with said first layer by means comprising flared ends on one of said layers extending over the ends of said other layer.

9. A transformer comprising: a wound core having an opening; an electrical winding toroidally wound on said core; an encapsulating first layer of electrical insulating material molded about said core and winding; a thin film of a mold release agent adhering to the surface of said first layer within said core opening; a ceramic tubular bushing coaxially extending through said core opening and spaced from said film and a second layer of insulating material molded into the remaining soace between said bushing and said release agent film, said second layer interlocked with said first layer by means comprising flared ends on one of said layers extending over chamfered end portions of said other layer.

References Cited in the file of this patent UNITED STATES PATENTS 2,804,577 Roth Aug. 27, 1957 2,920,297 Spicer Jan. 5, 1960 FOREIGN PATENTS 498,756 Great Britain Jan. 9, 1939 

1. A TRANSFORMER COMPRISING: A CORE HAVING AN OPENING; AN ELECTRICAL WINDING LINKED WITH SAID CORE; A FIRST LAYER OF ELECTRICAL INSULATING MATERIAL ENCAPSULATING SAID CORE AND WINDING; A BUSHING EXTENDING THROUGH SAID CORE OPENING; AND A SECOND LAYER OF INSULATING MATERIAL MOLDED IN CONTIGUOUS RELATIONSHIP WITH SAID BUSHING AND THE PORTION OF SAID FIRST LAYER ENCIRCLING SAID CORE OPENING. 